1. Field of the Invention
The present invention relates to a sheet transport apparatus in various printers, copiers, facsimiles and the like, and more specifically, to a driving mechanism of transport rollers for transporting a sheet to a sheet processing position at a constant velocity.
2. Description of Related Art
Generally, it is configured in copiers, facsimiles, printers and scanners that a sheet is transported to a predetermined processing position, and undergoes processing for reading the image, forming the image or the like in the processing position. Then, these apparatuses are provided with a sheet transport mechanism for sequentially separating sheets on a sheet basis to feed to a processing position.
For example, a scanner is provided with a sheet transport apparatus for feeding an original document sheet onto the platen to read the original document sheet. As this kind of scanner, the so-called sheet-through reading type has been known in which a sheet image of the moving original document sheet is read with a rested read optical series.
The sheet-through reading type is to read an original document sheet while moving the sheet to a predetermined reading position, therefore is short in processing time of the original document sheet, while providing a sheet transport mechanism with a simplified structure and the low number of components, and has the advantage enabling low cost to be actualized.
A sheet transport apparatus provided in this sheet-through reading type scanner is provided with a paper feed tray to mount original document sheets, a feeding roller for feeding the original document sheets on the paper feed tray, a paper feed roller and friction separating member for separating the fed original document sheets into a sheet to feed, a resist roller pair for aligning the front end of the original document sheet by causing the front end of the separated single sheet to hit the resist roller pair, and sending the sheet to a reading position, and a carry-in roller pair and carrying-out roller pair respectively provided on the upstream side and downstream side of the reading position to receive the original document sheet from the resist roller pair and pass the sheet through the reading position at a constant velocity (for example, Japanese Laid-Open Patent Publication No. 2003-125157).
Herein, in this type of sheet transport apparatus, as a transmission mechanism for transferring driving to each roller from a driving motor, used are a plurality of gears, pulleys, timing belts and the like. As a method of coupling the rotary shaft of each roller to the gear, pulley and the like, for example, such a method has been known that an end portion of the shaft is cut in the shape of a D, while fit holes of the corresponding gear and pulley are also formed in the shape of a D, and that the D-cut portion of the shaft is press-fitted to the D-shaped fit hole of the gear or pulley and fastened. This coupling method enables reliable transmission to the shaft without impairment of rotary torque and is excellent in this respect (for example, Japanese Laid-Open Patent Publication H07-320342).
However, in the aforementioned coupling method, since the shaft is press-fitted to the gear or pulley, the periphery of the gear or pulley expands. Particularly, since the expansion coefficient in the outer portion corresponding to the arc portion in the D-shaped fit hole is higher than the outer portion corresponding to the parallel portion, the periphery of the pulley becomes deformed after press-fitting to the shaft. In this way, the circularity error occurs due to misalignment of center axes of the shaft and gear or pulley, or local deformation of the pulley, and fluctuations occur in rotation transmission (velocity).
Then, such fluctuations in velocity do not cause any problems in normal transport of sheets, but when the velocity fluctuations occur in the carry-in roller and carrying-out roller for passing the original document sheet through a reading position at a constant velocity, fluctuations and distortion clearly appear in the read image, resulting in failure in the read image.
Particularly, in using a three-line color sensor provided with filters of R, G, B and three line sensors of R, G, B, respectively, since the same position of an original document sheet being transported is scanned and read sequentially by each line sensor while varying the time, and each scanned and read image data is subsequently composed, even a minute deviation in the scanning position of each line sensor with respect to the original document sheet causes a color displacement and pitch fluctuation in the image and becomes a serious problem.
The present invention was carried out in view of the aforementioned circumstances, and it is an object of the invention to provide a sheet transport apparatus for preventing the velocity of a sheet from fluctuating in the processing position of the sheet, and enabling excellent image processing.